This invention concerns peptidomimetic isoprenyl transferase inhibitor compounds useful in the treatment of human cancers.
Ras oncogenes are prevalent in over 20% of all human cancers. The compounds of the invention inhibit the posttranslational processing of ras proteins, thereby inhibiting ras protein function.
Ras proteins are present in all cell types, and are thought to take part in normal cellular signal transduction mechanisms. Ras mutations are thought to cause hyperproliferation of cells; mutated ras genes are known as oncogenes. In particular, ras oncogenes are found in approximately 30% of all lung cancer, 30% of all myeloid leukemia, 50% of all colorectal carcinoma, and 90% of all pancreatic carcinoma. Barbacid, M., Ann. Rev. Biochem., 56:779 (1987), Bos, J. L., Cancer Res. 49:4682 (1989). Examples of ras mutations include H-ras, K-ras, and N-ras.
Like other members of the superfamily of small GTP-hydrolyzing proteins, ras-encoded proteins, both normal and mutated, require post-translational processing for cell membrane association and biological activity. Maltese, W. A., FASEB Journal, 4:3319 (1990), Hancock, J. F. et al., Cell, 57:1167 (1989).
The post-translational processing of ras proteins is signaled by a short carboxyl-terminus consensus sequence, known as the CAAX box. This sequence signals which of two isoprenyl groups, farnesyl or geranylgeranyl, is to be attached to ras proteins by cellular enzymes. A farnesyl group is a 15 carbon isoprenyl group, while a geranylgeranyl group is a 20 carbon isoprenyl group. Isoprenyl groups are multimers of isoprene, a 5 carbon compound. For farnesylated proteins, such as ras, lamin B, and .gamma.-transducin, C is cysteine, A is an aliphatic amino acid, and X (the carboxyl-terminal amino acid) is methionine, serine, or glutamine. Geranylgeranylated proteins such as Rap, Rho and other small GTP-binding proteins, have similar CAAX sequences in which X is usually leucine, or occasionally is phenylalanine. In vivo, ras proteins are preferentially farnesylated.
Post-translational processing of the ras-encoded protein includes at least three steps. First, reaction with farnesyl pyrophosphate attaches a farnesyl group to the Cys residue on the sulfhydryl side chain. Second, a specific protease cleaves the three carboxy-terminal amino acids. Third, the carboxylic acid moiety of the now-terminal cysteine is methylated to a methyl ester. The farnesyl transferase enzyme (FTase) mediates the attachment of the farnesyl group to a protein. The geranylgeranyl transferase I enzyme (GGTase I) mediates the attachment of the geranylgeranyl group to a protein. Post-translational processing, particularly farnesylation, of ras proteins is critical for in vivo ras protein function. Among other things, farnesylation of ras oncogene products is known to be essential for ras-induced cellular transformation. Cox, A. D. and Der, C. A. Critical Rev. in Oncogenesis, 3 (4) 365-400 (1992). Upstream of FTase, farnesylation of a ras protein can be inhibited by mevalonate synthesis inhibitors such as lovastatin or compactin, which are HMG-CoA reductase inhibitors. Direct inhibition of FTase by short peptides or peptide-like substrates has also been demonstrated. Since ras proteins mediate the transformation of normal cells to cancer cells in many human cancers, compounds which inhibit prenylation will, therefore, inhibit the growth of ras-related cancers.